Circuits for voltage level translation are utilized in a variety of applications. Level translator circuits are employed to allow circuits operating at different power supply potentials to communicate with one another. Typically, the area, power and performance of the translator circuit are critical to the operations of each of the different circuits.
FIG. 1 is a simple block diagram of a level translator system 10. The system 10 includes a level translator circuit 12 coupled between transmitting circuit 14 and downstream receiving circuit 16. In this embodiment, Vddl circuit 14 is coupled to a lower voltage supply and Vddh circuit 16 is coupled to a higher voltage supply. Level translation is required only when a circuit on a lower voltage supply interfaces with one on a higher voltage supply. The level translates due to the voltage difference between Vddl circuit 14 and Vddh circuit 16.
It is important that a level translator circuit operate efficiently, utilize minimal power, and translate from one voltage supply potential to another as quickly as possible. To conserve standby or leakage power it is important to be able to turn off the transmitting circuit, i.e. circuit 14. It is also important to be able to provide a valid Vddl and Vddh level and minimize leakage current at all times, even when the transmitting circuit 14 is disabled. For a more detailed description of this issue, refer now to the following discussion in conjunction with the accompanying figures.
FIG. 2 illustrates a first embodiment of a conventional level translator circuit 100 coupled between distinct power supplies. In this circuit, cross-coupled pfet transistors 102 and 106 connected to the Vddh circuit 16′ are used in conjunction with pull-down nfet transistors 104 and nfet transistor 108 and an inverter 110 which is connected to the Vddl supply. The circuit 100 operates as follows: For propagation of a logical ‘0’ from the Vddl circuit 14′, transistor 104 is off, the inverter 110 produces a logical ‘1’ at node-2 in the form of Vddl volts, which then turns on nfet transistor 108, driving node-Z to a logical ‘0’, which in turn causes pfet transistor 102 to turn on, thereby raising node-1 to Vddh volts, which in turn causes pfet transistor 106 to turn off. Since the gate of nfet transistor 106 is at Vddh and the source of pfet transistor 106 is also at Vddh, there is no leakage. This circuit is non-inverting.
For propagation of a logical ‘1’ from the Vddl circuit 14′, nfet transistor 104 is on, resulting in node-1 being drawn toward a logical ‘0’. The inverter 110 produces a logical ‘0’ at node 2 in the form of 0 volts, which results in nfet transistor 108 turning off. Since node-1 is being drawn to 0 volts, pfet transistor 106 is now on, driving node Z to a logical ‘1’ in the form of Vddh volts, which in turn reinforces the node 1 potential of ‘0’ by turning off pfet transistor 102.
Now, if Vddl circuit 14′'s power were turned off, e.g., set to 0 volts to conserve power, while Vddh remained active, the output levels emanating from the Vddl circuit 14′ would be 0 volts. In addition, the output of the inverter 110 within the level translator would also be at 0 volts. When the Vddl supply is active, differential levels are supplied to the nfet transistors 104 and 108 comprising the level translator. Now with the Vddl supply being cut off, both nfet transistors 104 and 108 now receive the same 0 volts. This results in the output, node Z and node 1 achieving Vddh−|Vtp|. This will result in leakage in the Vddh circuit 16′ that is driving down stream.
FIG. 3 illustrates a second embodiment of a conventional level translator circuit 200 coupled between distinct power supplies. In this configuration, the inverter is eliminated and the source of nfet transistor 204 is connected to the gate input of the nfet transistor 208, which is connected to the output of the Vddl circuit 14″. Also, the gate of nfet transistor 204 is connected directly to the Vddl supply. This circuit 200 operates as follows: For propagation of a logical ‘0’ from the Vddl circuit 14″, nfet transistor 204 is on and nfet transistor 208 is off, thereby relinquishing control of node Z. Since nfet transistor 204 is on, node 1 is now at 0-volts turning on pfet transistor 206, raising node-Z to Vddh volts, which in turn shuts off pfet transistor 202, which reinforces the node 1 level of 0 volts. Also note that this circuit configuration is inverting.
For propagation of a logical ‘1’ from the Vddl circuit 14″, nfet transistor 204 is on until the voltage at node 1 can rise to Vddl−Vtn. This voltage rise begins to shut off pfet transistor 206. Nfet transistor 208 is now active and pulling node-Z low, which in turn activates pfet transistor 202, which raises the node 1 potential to Vddh, which in turn shuts off the leakage from pfet transistor 206. Once again, if Vddl were turned off, e.g., set to 0 volts to conserve power while Vddh remained active, all levels emanating from the Vddl circuit 14″ would be 0 volts.
What is needed therefore is a means to provide a downstream voltage level out of the level translator when the transmitting voltage potential circuit is shut off. The present invention addresses such a need.